Diamond single crystal substrate manufacturing method and diamond single crystal substrate

ABSTRACT

A diamond single crystal substrate manufacturing method for growing by vapor-phase synthesis a single crystal from a diamond single crystal seed substrate, comprising etching away by reactive ion etching, prior to single crystal growth, at least 0.5 μm and less than 400 μm, in etching thickness off the surface of the seed substrate which has been mechanically polished, thereby removing from the surface of the seed substrate the work-affected layers caused by mechanical polishing; and growing then a single crystal thereon. The manufacturing method provides a diamond single crystal substrate having a high quality, large size, and no unintentional impurity inclusions, and suitable for use as semiconductor materials, electronic components, optical components or the like.

RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.11/032,176, filed Jan. 11, 2005, claiming priority of JapaneseApplication Nos. 2004-009047, filed Jan. 16, 2004; 2004-081815, filedMar. 22, 2004; and 2004-322048, filed Nov. 5, 2004, the entire contentsof each of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a diamond single crystal substratemanufacturing method and a diamond single crystal substrate, especiallya diamond single crystal substrate manufacturing method and a diamondsingle crystal substrate, having a high quality, large size and nounintentional impurity inclusions, and suitable for use as semiconductormaterials, electronic components, optical components, or the like.

2. Description of the Prior Art

The many outstanding properties of diamond, such as high thermalconductivity, high electron/hole mobility, high dielectric breakdownfield, low dielectric loss and wide bandgap, make it an unparalleledsemiconductor material. In recent years, in particular, ultravioletlight emitting devices that take advantage of diamond's wide bandgap,and field effect transistors having excellent high frequencycharacteristics, etc. have been developed. Thanks to its transparencyextending from the ultraviolet to the infrared region, diamond showspromise as well as a material for optical components.

In order to use diamond as a semiconductor, it is necessary to make itinto high-quality, large-size single crystal substrates, as with othersemiconductor materials. Diamond single crystals are obtained at presentchiefly by high-temperature high-pressure synthesis methods, whichresult in superior crystallinity compared with natural single crystals,but that suffer from the inclusion of nitrogen impurities in thecrystals unless special growth conditions are used. A substratecontaining nitrogen is difficult to use on its own as a semiconductorsingle crystal, for which reason seed substrates are often used forgrowing diamond single crystals using a vapor-phase synthesis methodthat reduces impurity inclusions (see for instance, Japanese PatentPublication No. 2003-277183A).

Also, the size of the diamond single crystals obtained by thehigh-temperature high-pressure synthesis methods is limited to a gradeof 1 cm, and it is difficult to achieve large-size single crystals withdiameters of not smaller than 10 mm. Research is being conductedtherefore in order to manufacture a larger surface area single crystalsby vapor-phase synthesis, wherein the single crystals above are used asseed substrates. For instance, Japanese Patent Publication No. 03-75298Adiscloses a method for manufacturing diamond single crystals byarranging the orientations of a plurality of single crystal diamonds inapproximately the same direction and then growing a diamond thereon byvapor-phase synthesis thereon. For obtaining large-size diamond singlecrystals using such a method, Japanese Patent Publication No. 07-17794Adiscloses that, by controlling the crystal orientation, spacing andheight of a plurality of single crystal diamonds and restricting thegrowth temperature to an adequate range in order for homoepitaxialgrowth to be maintained up to a predetermined thickness, a high-purityvapor phase synthesis is achieved that allows providing large-sizediamond single crystals for optical or semiconductor use, with 15 mm orlarger diameters and having a good crystallinity, supported byultraviolet transparency near the 250 nm wavelength region, X-rayrocking curves with a half-width value of 100 seconds or less, Ramanscattering spectra with a half width of 2 cm⁻¹, etc.

As described in Japanese Patent Publication No. 2003-277183A, whendiamond single crystals are grown by vapor-phase synthesis on a diamondsingle crystal substrate obtained by a high-pressure synthesis process,residual stresses accumulate in the vapor-phase growth layer. Residualstresses remaining in the vapor-phase growth layer are problematic inthat the associated crystal strain may alter semiconductor propertiessuch as bandgap, mobility, etc. These phenomena are not solved by thevapor-phase growth of diamond single crystals from seed substrates 100μm or less thick, as disclosed in Japanese Patent Publication No.2003-277183A. Also, diamond single crystal substrates obtained byformation of thick films having a thickness of 100 μm or more byvapor-phase synthesis may threaten with substrate cracking due toaccumulated stresses. Owing to the increased probability of substratecracking entailed by substrates having a larger size (larger surfacearea, thicker film), the above problems in obtaining large-size diamondsingle crystal substrates, as described in particular in Japanese PatentPublication Nos. 03-75298A and 7-17794A, are not essentially solvedusing a method as described in Japanese Patent Publication No.03-75298A, wherein large-size single crystal substrates are obtained byintegrating in a single unit substrates consisting of an arrangement ofa plurality of high-pressure phase substances having essentiallymutually identical crystal orientations and acting as vapor-phase growthnuclei upon which is then grown a single crystal by vapor phasesynthesis, or using a method as described in Japanese Patent PublicationNo. 7-17794A, wherein large-size single crystal substrates are obtainedby controlling the single crystal growth conditions.

Also, the practical implementation of large-size diamond homoepitaxialgrowth by conventional techniques such as those described in JapanesePatent Publication Nos. 03-75298A and 7-17794A is plagued by problems ofunintentional impurities becoming enclosed in the crystal. Whenunintentional impurities become trapped in the crystal, an impuritylevel is reached which is incompatible with the targeted use assemiconductor, etc. and that impairs not only its use as a semiconductorsubstrate, but restricts as well, due to changes elicited in the opticalproperties of the crystal, its use as an optical substrate. Elementsenclosed as impurities in diamond single crystals grown homoepitaxiallyby chemical vapor-phase synthesis include mainly hydrogen, silicon,nitrogen, boron, etc.; among these, nitrogen impurities introducedunintentionally in the crystal are the most influential as regardssemiconductor and optical properties. Nitrogen is the element mostdifficult to control as an impurity, since it is an essential componentof the atmosphere and is therefore present in non-negligible amountsinside vacuum vessels. Thus in order to manufacture diamond singlecrystal substrates for semiconductor or optical use by vapor-phasesynthesis it is vital to assess and control nitrogen impurities;however, at present it is extremely difficult to predict semiconductorand optical properties by regulating the amount of impurity inclusionsthrough a strict control of the factors that affect their uptake. Thusfar, methods for obtaining diamond single crystals by heteroepitaxialmethods result in large numbers of crystal defects and therefore in aninsufficient quality for warranting use as optical or semiconductorsubstrates.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome the above shortcomingsand to provide a diamond single crystal substrate manufacturing methodand a diamond single crystal substrate having a high quality, largesize, and no unintentional impurity inclusions, and suitable for use assemiconductor materials, electronic components, optical components, orthe like.

In order to solve the above problems, the present invention is asfollows:

(1) A diamond single crystal substrate manufacturing method by growing asingle crystal from a diamond single crystal seed substrate byvapor-phase synthesis, the method comprising etching away by reactiveion etching, prior to single crystal growth, at least 0.5 μm and lessthan 400 μm, in etching thickness, off a surface of the seed substratewhich has been mechanically polished, and growing then a single crystalthereon.

(2) The method according to the above (1), comprising etching away,prior to single crystal growth, at least 50 nm, in etching thickness,off side faces of the seed substrate, and growing then a single crystalthereon.

(3) The method according to any one of the above (1) or (2), wherein theetching gas in the reactive ion etching is a mixture gas of oxygen andcarbon fluoride.

(4) The method according to any one of the above (1) to (3), wherein anetching pressure in the reactive ion etching is at least 1.33 Pa and nomore than 13.3 Pa.

(5) The method according to any one of the above (1) to (4), wherein thesubstrate temperature during etching in the reactive ion etching is 800Kor less.

(6) A diamond single crystal substrate grown from a diamond singlecrystal seed substrate by vapor-phase synthesis, wherein the diamondsingle crystal substrate is obtained by etching away by reactive ionetching, prior to single crystal growth, at least 0.5 μm and less than400 μm, in etching thickness, off a mechanically polished surface of theseed substrate, and by growing then a single crystal thereon.

(7) The diamond single crystal substrate according to the above (6),wherein the diamond single crystal substrate is obtained by etchingaway, prior to single crystal growth, at least 50 nm, in etchingthickens, off side faces of the seed substrate, and growing then asingle crystal thereon.

(8) The diamond single crystal substrate according to the above item (6)or (7), wherein the diamond intrinsic Raman shift by Raman spectroscopyof a substrate surface after single crystal growth falls within adeviation of 0.5 cm⁻¹ from 1332 cm⁻¹, which is the standard Raman shiftof a strain-free diamond.

(9) The diamond single crystal substrate according to any one of theabove (6) to (8), wherein a hole mobility of a hydrogenated surfaceconductive layer of the diamond single crystal substrate at normaltemperature as obtained by Hall measurement is 900 cm²/V·sec or more.

(10) The diamond single crystal substrate according to any one of theabove (6) to (9), wherein in a cathodoluminescence spectrum measuredacross all the faces of the diamond single crystal substrate at ameasurement temperature of 40 K or less, the emission peak intensity at575 nm is at least 2 times and no more than 10 times as high as thehighest peak intensity among peak intensities at any arbitrarywavelength within a range of from 200 nm to 900 nm (excluding theemission peak intensity at 575 nm) and a background intensity, and thepeak full-width at half-maximum at 575 nm is 2.5 nm or less.

(11) The diamond single crystal substrate according to anyone of theabove (6) to (9), wherein in a photoluminescence spectrum measuredacross all the faces of the diamond single crystal substrate at ameasurement temperature of 40K or less using an excitation light sourcehaving a wavelength of 514.5 nm, the emission peak intensity at 575 nmis at least 2 times and no more than 10 times as high as the highestpeak intensity among peak intensities at any arbitrary wavelength withina range of from 500 nm to 900 nm (excluding the emission peak intensityat 575 nm and a peak intensity at the excitation wavelength and Ramanpeak intensities caused by diamond lattice vibration) and a backgroundintensity, and the peak full-width at half-maximum at 575 nm is 2.5 nmor less.

(12) The diamond single crystal substrate according to any one of theabove (6) to (11), wherein the nitrogen concentration in the singlecrystal is 10 ppm or less.

(13) The diamond single crystal substrate according to any one of theabove (6) to (12), wherein the diameter of the diamond single crystalsubstrate is 10 mm or more.

(14) A diamond single crystal seed substrate for growing thereon adiamond single crystal by vapor-phase synthesis, wherein at least 0.5 μmand less than 400 μm, in etching thickness, are etched away by reactiveion etching off a mechanically polished surface of the seed substrate.

(15) The diamond single crystal seed substrate according to the above(14), wherein at least 50 nm, in etching thickness, is etched away byreactive ion etching off side surfaces of the seed substrate.

The term “surface” of the substrate in the above is also mentioned as“main face”. Throughout the present specification, the etching amount isindicated by etching thickness unless otherwise specified.

The method for manufacturing a diamond single crystal substrateaccording to the present invention allows manufacturing high-quality andstrain-free large-size diamond single crystal substrates forsemiconductor materials, electronic components, optical components, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a diamond single crystal seed substratebefore etching used in the present invention.

FIG. 2 is a side view of a seed substrate after etching used in thepresent invention.

FIG. 3 is a side view of an obtained diamond single crystal substrateaccording to the present invention.

FIG. 4 is a side view of the diamond single crystal substratemanufactured in Comparative Example 1.

FIG. 5 is an example of the cathodoluminescence (CL) spectrummeasurement in an example of the present invention.

FIG. 6 is the transmission spectrum of an example of the presentinvention.

FIG. 7 is the transmission spectrum of Comparative Example 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is explained next.

The present inventors analyzed in detail the phenomenon of stressaccumulation, occurring during vapor-phase growth of diamond singlecrystals on diamond single crystals used as seed substrates, using amicroscopic Raman spectrometer allowing the measurement oftwo-dimensional surface distributions. As a result, the inventorsdiscovered that the portions of single crystal grown from areas of theseed substrate with originally numerous mechanical polishing flawsdeviated from 1332 cm⁻¹, which is the standard Raman shift of diamond,within several cm⁻¹. Raman shift is caused by the natural vibrationfrequency of the crystal lattice; herein deviations from the intrinsicshift of diamond are used to identify strain in regions where crystallattice distances are narrower or wider than normal. It was thus foundthat the portions of single crystal grown from areas of the seedsubstrate having originally numerous mechanical polishing flaws wereareas with a higher than normal strain.

Next, the aforementioned areas of the seed substrate surface havingnumerous mechanical polishing flaws were cut by a focused ion beam andwere inspected under transmission electron microscopy. The resultsshowed a disarray of the diamond's crystallinity in the vicinity of thepolished surfaces of the areas with the larger number of mechanicalpolishing flaws, with partial non-crystalline layers. Furthermore,electron beam diffraction images obtained simultaneously showed, in theaforementioned areas, ring-shaped spreading images in addition to theintrinsic point images of diamond single crystal lattice, indicating thepresence of non-crystalline diamond and polycrystalline diamond whichare different from single crystal diamond, as well as unevenlydistributed dislocations/defects, etc. (hereinafter work-affectedlayers). The marked presence of work-affected layers in the areas withmore mechanical polishing flaws suggests that such work-affected layersare induced during mechanical polishing.

The side faces of the seed substrate above were inspected in the sameway and showed also identical work-affected layers. These work-affectedlayers in the side faces of the seed substrate were observed not only inpolished seed substrates but in seed substrates shaped by laser cuttingas well, an indication that work-affected layers are also affected bylaser processing.

Methods for identifying the impurities/defect levels in crystals includecathodoluminescence (hereinafter CL) spectrometry and photoluminescence(hereinafter PL) spectrometry. As a result of a diligent research of therelationship between CL and PL distributions, composition of the infusedgas, and crystallinity of vapor-phase synthesized single diamondcrystals, the inventors found that the single crystal diamond preferablymeets the following conditions. Specifically, the measured CL spectrumof a diamond single crystal cooled to a low temperature of 40K or lessshould have an emission peak intensity at 575 nm is at least 2 times andno more than 10 times as high as the highest peak intensity among peakintensities at any arbitrary wavelength within a range of from 200 nm to900 nm (excluding the above emission peak intensity at 575 nm) and thebackground intensity, and a peak full-width at half-maximum at 575 nm of2.5 nm or less. The emission peak at 575 nm of the PL spectrum measuredwith an excitation light source having a wavelength of 514.5 nm isconstrained in the same way as for the CL spectrum. Peak intensities aredefined herein as the effective peak intensity resulting fromsubtracting the background value from the maximum value of thecorresponding peak, and are usually obtained through aGaussian-Lorentian fitting.

Light emission at 575 nm in the PL and CL of diamond is called the N-Vcenter, and is believed to be caused by links between nitrogenimpurities and lattice vacancies (cf. Rep. Prog. Phys., Vol. 42, (1979)P. 1605). If during the growth of the single crystal the vapor-phasecontains nitrogen, the latter will be mixed into the single crystal andwill cause emission at 575 nm. Regarding specimens in which peakseparation was difficult, since peaks widened in the measurements ofCL/PL at normal temperatures, the present inventors carried out themeasurements of those specimens at a low temperature of 30K andsucceeded thereby in observing different peaks for different levels.This revealed that peak intensity at 575 nm is intimately related to thedegree of crystal impurities/defects, which are in turn correlated withsemiconducting and optical properties, and can be used therefore as abenchmark for regulating crystallinity. As a result, the unnecessarilystrict nitrogen impurity controls of conventional technology can nowgive way to diamonds of adequate crystallinity being obtainable withsuch controls kept to a minimum. The nitrogen impurity concentration indiamond single crystals thus obtained is preferably 10 ppm or lessrelative to carbon atoms. The present inventors noticed that a number ofnitrogen atoms below that value yielded semiconductor/optical substrateswith no problems in practice. Diamond single crystal having a diameterof 10 mm or more and satisfying the aforementioned CL and PL criteriahold much promise of becoming large-size single crystal substratesespecially suitable for optical use. In the present invention, diameterrefers to the length of the longest straight line that can be drawn in asingle crystal of a given size and shape.

The present invention, based on the above findings, comprises etchingaway by reactive ion etching (hereinafter RIE), prior to single crystalgrowth, at least 0.5 μm and less than 400 μm off the surface of the seedsubstrate obtained by mechanical polishing, and then growing a singlecrystal thereon. Since the surface of the single crystal seed substrateused for vapor-phase growth is polished mechanically, the polishedsurface contains work-affected layers with metallic impurities and/orprocessing defects, etc., that are disadvantageous for single crystalvapor-phase growth. High quality diamond single crystals object of thepresent invention can be obtained by conducting reactive ion etchingprior to the step of single crystal growth, because this pre-growthreactive ion etching removes these work-affected layers. Preparing seedcrystals free of the aforementioned work-affected layers should sufficeto curb the occurrence of strain in the diamond single crystals duringvapor-phase growth; however, in view of their origin, removing suchwork-affected layers by mechanical polishing is fraught withdifficulties.

Non-mechanical diamond processing includes various conventionalprocesses, for instance RIE etching, microwave plasma etching, ECRplasma etching, ion beam etching, etc. Among these non-mechanicalprocesses alternatives, only RIE methods can tackle successfully all theaspects of processing speed, processing area, surface roughness afterprocessing, formation of damaged layers upon etching, etc. RIE is ahigh-speed smooth method that allows removing only work-affected layersfrom the seed substrate without damaging the latter. Following RIE,vapor-phase growth of single crystals allows obtaining high-quality andstrain-free large-size diamond single crystal substrates.

The RIE according to the present invention may be carried out usingknown methods. In the present invention may be used for instance methodsusing capacitively coupled plasma (CCP) wherein a high-frequency powersource is connected to electrodes arranged facing one another inside avacuum vessel, or methods using inductively coupled plasma (ICP) whereina high-frequency power source is connected to a coil wound around avacuum vessel, or combinations of these methods.

Mixtures of oxygen and carbon fluoride are preferably used for theetching gas, with preferred etching pressures of 1.33 Pa or more and13.3 Pa or less. The above gas types and pressures allow removing onlythe work-affected layers, smoothly and at high speeds.

A suitable etching thickness of the seed substrate in the presentinvention is at least 0.5 μm and less than 400 μm, preferably at least 5μm and no more than 50 μm, and more preferably at least 10 μm and nomore than 30 μm. The shorter processing time of a thinner etchingthickness is advantageous for preserving surface smoothness. Thethickness of the work-affected layer of the seed substrate depends onthe type and strength of the polishing used. Most polishing is done withdepths of less than 0.5 μm, but occasionally depths of about 10 μm canbe reached locally, causing portions grown from such areas to exhibitinferior semiconductor properties. Conversely, a thicker etching depthrequires a longer etching time and may result in increasedetching-induced surface roughness, which may give rise to a degradationin crystallinity during the subsequent single crystal growth. A suitabletemperature of the substrate during etching is 800K or less, preferably600K or less. Carrying out the etching processing under the aboveconditions improves the crystallinity of the diamond single crystalsubstrate obtained thereafter by vapor-phase growth.

The side faces of the seed substrate according to the present inventionare etched away likewise by RIE, in order to remove from them preferablyat least 50 nm, more preferably at least 0.15 μm; in particular, whenthe side faces have also been mechanically polished, preferably 0.5 μmor more are removed from the side faces by etching. This allows reducingstrain in areas grown laterally from the side faces, especially in casesof expanded growth in the horizontal direction during thick-film growthof single crystals. There are RIE methods for side faces wherein etchingin the horizontal directing is performed simultaneously with the RIE ofthe seed substrate surface; however methods wherein only the side facesof a substrate standing up are etched are more effective as they allowcontrolling etching thickness individually.

Herein although any known method for diamond vapor-phase synthesis, suchas a microwave plasma CVD method, or a direct current plasma CVD method.may be used for single crystal vapor-phase growth, preferably amicrowave plasma CVD method is used from the viewpoint of growth rate,growth area and impurity inclusions.

In the diamond single crystal substrate according to the presentinvention obtained by the methods above, the fewer the work-affectedlayers on the seed crystal surface the better. In order to obtain ahigh-quality single crystal substrate by vapor phase synthesis using adiamond single seed crystal substrate according to the presentinvention, or to obtain a single crystal substrate at least 100 μmthick, the area density of work-affected layers in the etched seedsubstrate should be at most 1 occurrence per cm² (0.01 occurrence/mm²),and the ratio of the surface occupied by the work-affected layersrelative to the substrate should be 0.001% or less. The density andsurface area of these work-affected layers can be evaluated bytransmission electron microscopy inspection, as mentioned above, or bymeans of microscopic Raman spectroscopy, described below, growingultra-thin (e.g. thickness of 1 μm or less) diamond single crystals, orby any other means apart from these two.

After the growth of the diamond single crystal by vapor-phase synthesisaccording to the present invention, the surface of the resultant singlecrystal substrate can be evaluated for strain using a microscopic Ramanspectrometer. High-quality single crystals substrates can be achieved bykeeping the Raman shift caused by strain in the surface of the diamondsingle crystal substrate according to the present invention within adeviation of 0.5 cm⁻¹ from a Raman shift of 1332 cm⁻¹ corresponding to astrain-free diamond single crystal (standard shift).

The CL measurement conditions in the present invention do not require apre-measurement application of a conductive layer coating on the facesto be measured. Herein the voltages and currents of the electron beamare not limited to customary values that range respectively from 10 to30 kV and from 0.1 to 100 nA.

In the present invention, the wavelength of the excitation light sourceused in the PL measurement must be that of a 514.5 nm monochromaticlight, for which an argon gas laser is ordinarily used. The output ofthe excitation light source may have an arbitrary intensity, but whencollected by a microscope, etc., an excessive energy density mightdamage the specimens, for which reason measurements must be carried outwithin threshold levels. Unlike CL spectra, PL spectroscopy measures thelight source wavelengths (Rayleigh scattered light) and the Ramanscattered light caused by vibrations of the diamond lattice, though notfor comparison with PL peak at 575 nm.

The common CL/PL measurement conditions in the present invention mayinvolve measurements with crystals at a temperature of 40K or less, butalso any other temperature provided the CL/PL peak intensity conditionsaccording to the present invention are satisfied. In the presentinvention 30K was set as a standard temperature. Herein a 575 nm peakfull-width at half-maximum of 2.5 nm or less is desired, therefore aspectrum-measuring device having a wavelength resolution of 1.0 nm orless is preferable.

Examples of the present invention are described in detail below, withreference made to relevant accompanying drawings.

The diamond seed substrates used in Examples 1 and 2 of the presentinvention and Comparative Examples 1 to 3 will be explained first. Thediamond seed substrate used in these examples were prepared in the samemanner.

A diamond single crystal obtained by the high-temperature high-pressuremethod was used as a diamond seed substrate 3. The substrate was a plate4 mm long and wide and 0.5 mm thick with orientation {100} in the mainface 1 and the side faces 2. The main face 1 had been mechanicallypolished and in the side faces 2 the carbon layers had been removed bybichromic acid treatment after laser cutting/shaping. The surfaceroughness (Rmax) was 0.1 μm. The Raman shift distribution of the mainface was measured using a microscopic Raman spectrometer allowing themeasurement of two-dimensional surface distributions; the measurementresults centered around a Raman shift of 1332 cm⁻¹ corresponding to astrain-free diamond single crystal (hereinafter “standard shift”) with a0.1 cm⁻¹ shift deviation from that value (hereinafter “strain shift”,deviation from the standard shift). Furthermore, a seed substrateprepared separately, inspected under transmission electron microscopy,showed the presence of work-affected layers 4 on the main face of theseed substrate, as shown in FIG. 1.

Example 1

The main face and side faces of the above seed substrate were etched byRIE using CCP of conventional high-frequency interelectrode dischargetype. The etching conditions are listed in Table 1.

TABLE 1 High-frequency wave frequency 13.56 MHz High-frequency wavepower 300 W Chamber inner pressure 6.67 Pa O₂ gas flow rate 10 sccm CF₄gas flow rate 10 sccm

By etching the main face during 5 hours and, with the substrate standingup, all the side faces during 30 minutes each, under the conditionsshown in Table 1, 15 μm of the main face 1 and 1.5 μm of each of theside faces 2 of the seed substrate 3 were removed (FIG. 2). In FIG. 2, 5is the etched away layer. The roughness after etching, 0.1 μm, showed nochange relative to the roughness prior to etching. Also, a seedsubstrate prepared separately was etched under etching conditionsidentical to those listed in Table 1, then its surface after etching wasassessed by transmission electron microscopy, showing no work-affectedlayers.

Next, a diamond single crystal was grown by vapor phase on the seedsubstrate 6 after etching using a conventional microwave plasma CVDmethod. The growth conditions are given in Table 2.

TABLE 2 Microwave frequency 2.45 GHz Microwave power 5 kW Chamber innerpressure 1.33 × 10⁴ Pa H₂ gas flow rate 100 sccm CH₄ gas flow rate 5sccm Substrate temperature 900° C. Growth time 20 hours

The diamond single crystal was grown by the vapor phase synthesis methodby 200 μm on the seed substrate 6 under the growth conditions listed inTable 2 (FIG. 3). The Raman shifts of the diamond single crystal layer 7after growth was measured by a microscopic Raman spectrometer and showedstrain shifts within 0.1 cm⁻¹ for the entire grown surface. In order toassess semiconductor properties, the substrate was subjected to hydrogenplasma treatment and the thus hydrogenated surface conductive layer wasevaluated for the hole mobility at normal temperature by Hallmeasurement; herein high mobility values of 1000 cm²/V sec wereobtained. The results of this evaluation are given in Table 3.

Table 3 summarizes the values for etching thickness, surface roughnessafter etching, density of work-affected layers, maximum Raman strainshift after single crystal growth, and hole mobility by Hallmeasurement.

Comparative Example 1

In the present comparative example testing was performed under the sameconditions of Example 1 except that herein no etching of the seedsubstrate 3 was carried out. In this case, where the diamond singlecrystal was grown under the conditions of Table 2 without etching,crystal strain was measured in the single crystal areas grown fromwork-affected layers (FIG. 4). In FIG. 4, reference numeral 8 representsthe strain regions grown from the work-affected layers. The valuesobtained for maximum strain shift (2.5 cm⁻¹) by Raman spectroscopy andhole mobility (100 cm²/V·sec) were both inadequate for use as asemiconductor substrate.

Comparative Example 2

In the present comparative example testing was performed under the sameconditions of Example 1 except that herein the etching thickness of theseed substrate main face 1 was 0.4 μm and the etching thickness of theside faces 2 was 0.04 μm. Most of the work-affected layers were removedafter etching but parts of deep work-affected layers remained unchanged,without being etched. A diamond single crystal was grown on thissubstrate under the same conditions listed in Table 2, and as inComparative Example 1, crystal strain was measured also in the singlecrystal areas grown from work-affected layers. The values obtained formaximum strain shift (1.1 cm⁻¹) by Raman spectroscopy and hole mobility(220 cm²/V·sec), though improving on those of Comparative Example 1,were inadequate for use as a semiconductor substrate.

The evaluation results for Comparative Examples 1 and 2 are given inTable 3.

Example 2

The present example, wherein the etching thickness of the seed substratemain face 1 was 0.6 μm and the etching thickness of the side faces 2 was0.06 μm, involved a relatively thinner etching thickness.

Several seed substrates were used, among which in one case was measuredone work-affected layer after etching over a square substrate 4 mm wide,and in one case no such layer was measured (i.e. 0 to 1 occurrences/16mm²). Diamond single crystals were grown on these substrates under theconditions listed in Table 2, apparently with no crystal strain,although the measurement of the crystal surfaces by Raman spectroscopy,yielding a maximum strain shift of 0.3 cm⁻¹, did show a slight strain.The hole mobility of 910 cm²/V·sec was that of a relativelyhigh-mobility performance.

Comparative Example 3

In the present comparative example testing was performed under the sameconditions of Example 1 except that herein the etching thickness of themain face 1 was 450 μm and the etching thickness of the side faces 2 was45 μm.

The surface after etching was somewhat rough, with a Rmax value of 10.1μm. Though no work-affected layers were observed, some growth unevennesscaused by initial surface roughness was detected in the subsequentlygrown diamond single crystals. As a result, the values obtained formaximum strain shift (0.6 cm⁻¹) by Raman spectroscopy and hole mobility(410 cm²/V·sec) were inadequate for use as a high-performancesemiconductor substrate.

TABLE 3 After single After etching crystal growth Etching Work-affectedRaman thickness (μm) Surface layer density maximum Hole main sideroughness (occurrences/ strain mobility (cm²/ face face Rmax (μm) 16mm²) shift (cm⁻¹) V · sec) Example 1 15 1.5 0.1  0 0.1 1000 Comp. 0 00.1 20 2.5 100 Example 1 or more Comp. 0.4 0.04 0.1  3 1.1 220 Example 2Example 2 0.6 0.06 0.1 0-1 0.3 910 Comp. 450 45 10.1  0 0.6 410 Example3

Examples 3 to 5 Comparative Examples 4 to 5

Below are explained examples in which vapor-phase synthesized diamondsingle crystal substrates were obtained through homoepitaxial growth byvapor phase synthesis from diamond single crystal seed substratesobtained by the high-temperature high-pressure method.

The seed substrate was a plate 8 mm long and wide (diameter 11.2 mm) and0.5 mm thick with main face 1 and side faces 2 mechanically polished.The orientation was {100} for both the main face 1 and the side faces 2.First, the surface layer of the main face 1 of the seed substrate 3 wasetched away by reactive ion etching (RIE) of a conventionalhigh-frequency interelectrode discharge type (CCP). The etchingconditions are listed in Table 4.

TABLE 4 High-frequency wave frequency 13.56 MHz High-frequency wavepower 300 W Chamber inner pressure 6.67 Pa O₂ gas flow rate 10 sccm CF₄gas flow rate 10 sccm Substrate temperature 550 K.

0.6 μm of the seed substrate main face were removed after etching for 1hour under the conditions listed in Table 4.

Next, a diamond single crystal was homoepitaxially grown on thesubstrate 6 using a conventional microwave plasma CVD method. The growthconditions are given in Table 5.

TABLE 5 Microwave frequency 2.45 GHz Microwave power 5 kW Chamber innerpressure 1.33 × 10⁴ Pa H₂ gas flow rate 100 sccm CH₄ gas flow rate 15sccm H₂ and CH₄ gas purity not less than 99.9999% Nitrogen concentrationin the vapor 3 ppm phase Substrate temperature 1300 K. Growth time 30hours

The film deposition yielded a diamond single crystal substrate with avapor-phase synthesized single crystal layer 0.5 mm thick.

The vapor-phase synthesized single crystal layer of the diamond singlecrystal substrate was cut off by laser cutting, then the growth facesand the cut faces were polished and CL and PL measurements were carriedout under the conditions listed in Table 6 and Table 7, respectively.

TABLE 6 Measurement temperature 30 K. Electron acceleration voltage 15kV Electron current 17 nA Wavelength resolution 0.8 nm or less

TABLE 7 Measurement temperature 30 K. Wavelength of excitation lightsource 514.5 nm Output of the excitation light source 10 mW Wavelengthresolution 0.9 nm or less

FIG. 5 shows the CL spectrum measured under the conditions of Table 6.As can be seen in FIG. 5, a sharp peak at a wavelength of 575 nm wasmeasured among other peaks. The value (PA/PB) resulting from dividingthe emission peak intensity (PA) at 575 nm by the highest intensity (PB)among other peaks and the background (in this measurement at 588 nm) was4.86, while the full-width at half-maximum (PW) of PA was 1.03. The PLspectrum was obtained under the conditions of Table 7 in the same way asthe CL spectrum; herein PA/PB was 3.95, and PW=1.21.

The crystallinity of the diamond single crystal substrate obtained inthe present example was evaluated next. First, as an assessment ofsemiconductor properties, each substrate as a test specimen wassubjected to hydrogen plasma treatment and the thus hydrogenated surfaceconductive layer was evaluated for the hole mobility at normaltemperature by Hall measurement; the high mobility value of 1050cm²/V·sec obtained herein was adequate for semiconductor substrates.Next the amount of nitrogen impurities in the crystals wasquantitatively determined by secondary ion mass spectrometry, yielding asufficiently low value of 3.1 ppm relative to carbon atoms. Finallyoptical properties were evaluated on the basis of the transmissionspectrum in a wavelength region extending from 200 nm to 800 nm (FIG.6). As shown in FIG. 6, though some absorption caused by nitrogenimpurities can be observed at a wavelength of 270 nm, there istransmission down to the 225 nm wavelength corresponding to theintrinsic absorption end of diamond; the crystal has therefore adequateoptical properties. These results indicate that the diamond singlecrystal substrate of the present example is both a large-size and a highquality substrate.

Examples and comparative examples with modified single crystal growthconditions and changes in CL and PL are described next. High-puritysingle crystals were grown changing the H₂ and CH₄ gas purities shown inTable 4 in the preceding examples, and also the vacuum sealing methodwas modified in the growth process of Comparative Example 4. Except forthe concentration of nitrogen in the vapor phase, the growth conditionsand evaluation criteria were herein identical to those of the precedingexamples. The growth conditions and the crystallinity evaluation resultsare given in Table 8.

TABLE 8 Nitrogen Hole concentration CL PL mobility Nitrogen UV in thevapor P_(W) P_(W) (cm²/ impurities optical phase (ppm) P_(A)/P_(B) (nm)P_(A)/P_(B) (nm) V · sec) (ppm) properties Example 3  3 4.86 1.03 3.951.21 1050 3.1 Good (FIG. 6) Example 4  1 2.05 0.95 2.01 1.16 1100 0.9Good Example 5  10 9.98 2.26 9.64 2.48 900 9.4 Good Comp.  0.1 or less1.88 0.94 1.81 1.12 1300 0.1 Good Example 4 Comp. 100 10.9 2.51 10.32.67 300 32 Poor Example 5 (FIG. 7)

In the Examples 3, 4 and 5 in Table 8, growth was carried out changingonly the purity of the infused gases thus modifying the concentration ofnitrogen in the vapor phase. For both CL and PL, PA/PB ranged from 2 to10 and PW was 2.5 nm or less. The hole mobility of 900 cm²/V·sec or morewas a sufficient high mobility value for a semiconductor substrate. Thenitrogen impurities were small, with transmittance up to the ultravioletregion, thus affording satisfactory optical properties.

Next, in Comparative Example 4, a single crystal was grown lowering tothe limit the concentration of nitrogen in the vapor phase. A metal sealgasket was used in the vacuum seal member of the vacuum vessel and anultrapure gas (purity 99.99999% or more) was used to achieve a nitrogenconcentration of 0.1 ppm or less in the vapor phase. The single crystalobtained as a result showed a PA/PB for CL and PL of 1.9 or less andalso good semiconductor and optical properties, though not markedlydifferent from the characteristics observed in the invention Examples.This meant that the gas purity control applied in the presentComparative Example was unnecessarily strict.

In Comparative Example 5 the nitrogen concentration in the vapor phasewas increased. The single crystal obtained showed a PA/PB for CL and PLof more than 10, and a PW spreading to 2.5 nm or more. As shown in FIG.7, hole mobility dropped to ⅓ or less of the values in the Examples andnitrogen impurities increased, resulting in a deterioration of opticalproperties.

As demonstrated above, it was confirmed that the diamond single crystalseed substrates and the diamond single crystal substrates manufacturedby the method of the present invention as represented by the aboveexamples were desirably used as single crystal seed substrates andsingle crystal seed substrates for high-quality semiconductor materials.

The diamond single crystal substrates manufactured by the manufacturingmethod for diamond single crystal substrates according to the presentinvention are strain-free and have a high quality, with no unintentionalimpurity inclusions, which makes them suitable for use as large-sizediamond single crystal substrates for semiconductor materials,electronic components, optical components, etc.

1. A diamond single crystal substrate manufacturing method by growing asingle crystal from a diamond single crystal seed substrate byvapor-phase synthesis, said method comprising etching away by reactiveion etching, prior to single crystal growth, at least 0.5 μm and lessthan 400 μm, in etching thickness, off a surface of the seed substratewhich has been mechanically polished, and growing then a single crystalthereon.
 2. The method according to claim 1, comprising etching away,prior to single crystal growth, at least 50 nm, in etching thickness,off side faces of said seed substrate, and growing then a single crystalthereon.
 3. The method according to claim 1, wherein an etching gas insaid reactive ion etching is a mixture gas of oxygen and carbonfluoride.
 4. The method according to claim 1, wherein an etchingpressure in said reactive ion etching is at least 1.33 Pa and no morethan 13.3 Pa.
 5. The method according to claim 1, wherein the substratetemperature during etching in said reactive ion etching is 800K or less.6. A diamond single crystal substrate grown from a diamond singlecrystal seed substrate by vapor-phase synthesis, wherein the diamondsingle crystal substrate is obtained by etching away by reactive ionetching, prior to single crystal growth, at least 0.5 μm and less than400 μm, in etching thickness, off a mechanically polished surface of theseed substrate, and by growing then a single crystal thereon.
 7. Thediamond single crystal substrate according to claim 6, wherein thediamond single crystal substrate is obtained by etching away, prior tosingle crystal growth, at least 50 nm, in etching thickens, off sidefaces of said seed substrate, and growing then a single crystal thereon.8. The diamond single crystal substrate according to claim 6, wherein adiamond intrinsic Raman shift by Raman spectroscopy of a substratesurface after single crystal growth falls within a deviation of 0.5 cm⁻¹from 1332 cm⁻¹, which is the standard Raman shift of a strain-freediamond.
 9. The diamond single crystal substrate according to claim 6,wherein a hole mobility of a hydrogenated surface conductive layer ofthe diamond single crystal substrate at normal temperature as obtainedby Hall measurement is 900 cm²/V·sec or more.
 10. The diamond singlecrystal substrate according to claim 6, wherein in a cathodoluminescencespectrum measured across all the faces of the diamond single crystalsubstrate at a measurement temperature of 40K or less, an emission peakintensity at 575 nm is at least 2 times and no more than 10 times ashigh as a highest peak intensity among peak intensities at any arbitrarywavelength within a range of from 200 nm to 900 nm (excluding saidemission peak intensity at 575 nm) and a background intensity, and apeak full-width at half-maximum at 575 nm is 2.5 nm or less.
 11. Thediamond single crystal substrate according to claim 6, wherein in aphotoluminescence spectrum measured across all the faces of the diamondsingle crystal substrate at a measurement temperature of 40K or lessusing an excitation light source having a wavelength of 514.5 nm, anemission peak intensity at 575 nm is at least 2 times and no more than10 times as high as a highest peak intensity among peak intensities atany arbitrary wavelength within a range of from 500 nm to 900 nm(excluding said emission peak intensity at 575 nm, a peak intensity atthe excitation wavelength and Raman peak intensities caused by diamondlattice vibration) and a background intensity, and a peak full-width athalf-maximum at 575 nm is 2.5 nm or less.
 12. The diamond single crystalsubstrate according to claim 6, wherein the nitrogen concentration inthe single crystal is 10 ppm or less.
 13. The diamond single crystalsubstrate according to claim 6, wherein the diameter of the diamondsingle crystal substrate is 10 mm or more.
 14. A diamond single crystalseed substrate for growing thereon a diamond single crystal byvapor-phase synthesis, wherein at least 0.5 μm and less than 400 μm, inetching thickness, are etched away by reactive ion etching off amechanically polished surface of the seed substrate.
 15. The diamondsingle crystal seed substrate according to claim 14, wherein at least 50nm, in etching thickness, are etched away by reactive ion etching offside surfaces of the seed substrate.